Yes, the original relation controller has expansion restrictions. In the settings you need to enable "Use modificated OctoberCMS core files" (there are 3 files, including RelationController.php, where "protected $alias;" is replaced with "public $alias;").
These files are located in the "/linkonoid/backtofront/vendor/backtofront/modifed" folder (maybe I shouldn't have put them in the "vendor" folder), this is in case you change this files manually.
The code below is from the test server. Please note that you have at least 1 problem: "url" is not correct set. The component does not understand what alias, operation and Id to use. Lists must have 3 parameters ("/:alias?/:crud?/:id?")
The "testbtfplugin" plugin is completely authentic to the standard plugin and should work without this plugin in backend mode. The folder "plugins/linkonoid/backtofront/components/backtofront" contains modified partials that allow the correct display of standard widgets in the frontend (the "viewPath" parameter in the component settings).